This invention relates to porous PTFE and a process of producing same.
The outstanding characteristics of PTFE, such as its chemical inertness, its resistance to corrosion, its dielectric properties and its low frictional properties have resulted in this material being used in many diverse applications.
However certain of the inherent characteristics of PTFE produced by established methods--such as the void-free nature of the material and its inability to adhere to other materials without etching, abrading or otherwise treating the PTFE--means that it is not suitable for use in, for example, diffusion or filtration processes where porous materials are required.
In particular, liquid and gasous diffusion processes such as plasma phoresis, renal dialysis, desalination of water and blood oxygenation, utilise thin porous membranes as the control element, the non-porous nature of established PTFE products preventing their use in such processes. Most such membranes are manufactured either from cellulose and its derivatives, or from such materials as ceramics, asbestos, glass, porcelain and metals. In the former case, the membranes produced are relatively mechanically weak, particularly when produced to achieve high flow rates therethrough and often require the addition thereto of mechanical supports. In the latter case, the membranes, although mechanically strong, are not as chemically inert as PTFE. Other plastics materials have been used, but these are invariably less chemically inert than PTFE and may also require mechanical support.
It has been proposed to produce porous PTFE membranes by means of various specialist processes such as by plasma desposition, by conventional compression moulding but at pressures lower than those normally recommended for the production of a void-free material, by paste-forming extrusion techniques and subsequent expansion by stretching, or by incorporating within the PTFE thermally or chemically degradable materials which can be leached out during the process to leave the desired voids. However, such processes can require the use of complex and therefore expensive equipment and often result in the production of relatively weak membranes, or membranes which, because they are thin, are difficult to handle.